Merge mappings with equal SQL table and subject map

convert mappings which just project different columns from one table into one TripleMap, assuming this is safe (which I guess amounts to checking if the subjectMaps are equal).

So the two mappings (map:m-1, map:m-2) below (they only differ on y1/y2), should be trandformed into map:supermap, also below, which uses tt:table instead of R2RMLView, and contains multiple predicateObjectMap-s. The benefit is at least less duplication of the subjectMap.